ILM base libraries providing:

- Half is a class that encapsulates our 16-bit floating-point format.
- IlmThread is a thread abstraction library for use with OpenEXR and
  other software packages. It currently supports pthreads and Windows
  threads.
- Imath implements 2D and 3D vectors, 3x3 and 4x4 matrices, quaternions
  and other useful 2D and 3D math functions.
- Iex is an exception-handling library.
